The intestine is the central organ for uptake of fluids and nutrients, but it is also the largest immune organ. Nowhere in the body has the immune system evolved more regulatory mechanisms to allow for potent and effective immune protection while maintaining the integrity of the mucosal barrier. In mice, several subsets of mucosal regulatory immune cells have been identified. They include the CD8aa+ TCRap" intra epithelial lymphocytes (IEL), the mucosal CD8aa+plasmacytoid DCs (pDCs) and CD8aa+CD4+ TCRa|3+IEL. The conditions that lead to the differentiation of these mucosal regulatory cells and the mechanisms they employ to orchestrate immune regulation, are poorly defined or not known. In this study we propose to investigate and elucidate differentiation, function and cross communication of mucosal immune regulatory cells. In the first Aim we will define regulatory mechanisms engaged by the SP CD8aa+ IEL to communicate immune regulation to other cells. In the second Aim we will investigate the regulatory role of mucosal DCs, and in the third Aim will focus on the regulatory DP CD4CD8aa TCRafi* IEL. All of these mucosal regulatory cells uniformly express CD8aa. Based on our previous published findings supported by this grant, that CDScca can act as a modulator of activation signals leading to shifts in cytokine production, survival and differentiation of the triggered cell, we propose to further investigate in this study if CDSaa on these regulatory cells might play key roles for their function and/or survival. Mucosal regulatory cells, which allow for effective immune protection of the intestine without destruction of this vital organ, ultimately control the wellbeing of the whole organism. Understanding mucosal immune regulation will undoubtedly impact our knowledge to advance medical treatments not only for intestinal related diseases but for health improvement in general.